On top of that, new data shows that U.S. economic growth slowed at the end of last year as the economies of China and other nations have faltered.

So there's great anticipation for Yellen's appearance at a House Financial Services Committee hearing, where she will testify about the monetary policy and the economy in the first of two days on Capitol Hill.

Here's what to expect when the hearing begins at 7 a.m.

Talk of market mayhem

In her prepared testimony, Yellen is almost certain to address the recent stock market turmoil and indicate how much it concerns the Fed. If she doesn't, one of the committee members is sure to ask about it.

Federal Reserve policymakers on Wednesday said that U.S. economic growth slowed late last year and that they were “closely monitoring global economic and financial developments” amid recent stock market turmoil.As expected, Fed officials held their benchmark short-term interest rate steady at between...

Federal Reserve policymakers on Wednesday said that U.S. economic growth slowed late last year and that they were “closely monitoring global economic and financial developments” amid recent stock market turmoil.As expected, Fed officials held their benchmark short-term interest rate steady at between...

(Jim Puzzanghera and Don Lee)

When central bank policymakers opted to hold the so-called federal funds rate steady last month, they warned in their written statement that “global economic and financial developments may restrain economic activity somewhat” and probably would push down already-low inflation.

Yellen needs to say more than that but must be careful not to rattle investors further.

"Markets are tumultuous and anything she says could make things worse," said Mark Hamrick, senior economic analyst at Bankrate.com, a financial information website. "I think her primary concern right now is to do no harm.

The means she'll probably provide the same basic answer, with the words carefully chosen ahead of time, no matter how many different ways lawmakers try to get her to say more. The reaction of major stock indexes will help determine how well she threads the needle.

"She has demonstrated that she is adept at walking the fine line between saying too little and saying too much," Hamrick said. "But this might be the greatest challenge she’s faced yet."

March madness

Before the recent global economic troubles, most analysts expected the Fed to nudge the federal funds rate up another 0.25 percentage point in March. Now, many expect the Fed will wait to avoid making the situation worse by further tightening credit.

But with inflation extremely low, economist Bluford Putnam thinks Yellen will signal a March rate hike is unlikely.

“We don’t have any inflation, and if you don’t have any inflation you’re not going to want to raise rates,” said Putnam, who is managing director at CME Group, the futures market operator based in Chicago.

Going negative?

After holding the federal funds rate near zero since late 2008, central bank policymakers only inched up its target range by 0.25 percentage point.

Significant improvements in the labor market in 2014 and 2015 paved the way for the Fed's interest rate hike in December.

But wages have been slow to catch up and liberals have argued that the Fed shouldn't pull back too much from its stimulative policies until they do.

The other half of the mandate is to maintain stable prices. And inflation has been running well below the Fed's 2% annual target.

Yellen and other Fed officials said the key reason for low inflation is falling oil prices and the effects will be temporary.

With low inflation and sluggish wage growth, Democrats on the committee could press Yellen to delay future rate increases until there are signs inflation is rising.

Republican criticism

Most Congressional Republicans weren't happy with the Fed's unprecedented stimulus policies, which included seven years of a near zero interest rate and a bond-buying program that more than quadrupled the central bank's assets since 2008 to $4.5 trillion.